The present invention relates to fluoride glass fibers, and more particularly to fluoride glass optical fibers.
Fluoride glasses are non-oxide glasses. They are usually based on heavy metal fluorides such as ZrF.sub.4, HfF.sub.4 or BaF.sub.2. They also include glasses based on AlF.sub.3 and BeF.sub.2, as well as fluoro-phosphate-based glasses. Unlike oxide glasses, fluoride glasses have a relatively high tendency toward devitrification and have to be quenched rapidly from the melt to avoid crystal formation.
At present, glass-clad fluoride glass optical fibers are generally prepared from preform drawing. Fluoride glass preforms are fabricated from rotational casting and built-in casting techniques which are widely used nowadays. In the rotational casting approach, molten fluoride cladding glass is cast inside a metallic mold pre-heated at the glass transition temperature (Tg); the mold is then rotated and the melt solidifies into a concentric and uniform tube. Finally, the molten core glass which has a higher refractive index than the cladding glass is poured into the tube to form a fluoride glass preform having a wave guide structure. The preform is then drawn into fibers using a resistance or RF induction furnace. In the built-in casting method, the cladding tube is prepared by casting the cladding glass melt inside a metallic mold and subsequently upsetting the mold to allow the still molten glass to flow out of the mold; the core melt is then introduced into the tube as in the rotational casting technique.
The rotational casting and built-in casting techniques are limited to small preforms. Scaling up to large-site preforms for long-length fiber fabrication results in the formation of crystalline defects and bubbles inside the core of the preform. For a large-size preform, large amounts of core melt must be used and thus the heat dissipation through the wall of the cladding glass tube is relatively slow. The slow cooling of the core melt results in crystal formation within the core. Bubbles in the core represent an additional source of scattering defects. They are formed by trapped gas which originates from turbulence when the core melt is poured into the tube.
Similar scattering centers, namely crystals and bubbles, can be observed in polymer-clad fluoride glass fibers. Present techniques for fabricating polymer clad fluoride fiber consist of casting the melt inside a metallic mold to form a solid fluoride glass rod. The rod is then drawn into fiber which is coated in line with a low refractive index polymer which acts as an optical cladding. Scaling up to large-size rod again results in crystallization due to the slow quenching of the melt and bubble formation due to turbulence.